Dielectric device

ABSTRACT

The dielectric device includes a substrate, a lower electrode, a dielectric layer, and an upper electrode. The lower electrode is bonded onto the substrate. The dielectric layer is bonded onto the lower electrode. The dielectric layer is obtained through thermal treatment of a film layer formed by spraying of a powdery dielectric material and a fine-particulate metal. In the thus-formed film layer, the metal is dispersed in the matrix of the dielectric material. Thermal treatment of the film layer causes migration of the metal in the film layer. This metal migration causes a lower-electrode-adjacent portion and upper-surface-adjacent portion of the dielectric layer to have different metal contents.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 11/563,776,filed

Nov. 28, 2006, now abandoned, the entirety of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dielectric device including adielectric layer, and to a method for producing the dielectric device.

2. Description of the Related Art

A variety of dielectric devices of this type have conventionally beenknown. Generally, such a dielectric device includes, in addition to adielectric layer, a predetermined substrate, a lower electrode layer,and an upper electrode. The lower electrode layer is formed on thesubstrate. The dielectric layer is formed on the lower electrode layer.The upper electrode is formed on the dielectric layer. The dielectriclayer is formed by subjecting the lower-electrode-layer-formed substrateto film formation through, for example, the screen printing process, thegreen sheet process, aerosol deposition method, or powder jet depositionmethod.

The screen printing process is a technique in which a substrate iscoated, through screen printing, with a slurry prepared by dispersingceramic powder in a solvent containing an organic binder, and theresultant coating film is sintered at a high temperature of 900° C. orhigher, to thereby form a dielectric layer. The green sheet process is atechnique in which a thick film having a predetermined thickness isformed from the aforementioned slurry, followed by drying, therebyyielding a green sheet; the green sheet is subjected to a predeterminedmachining process such as cutting or drilling; and the resultant greensheet is sintered at a high temperature in a manner similar to thatdescribed above, to thereby form a dielectric layer. Aerosol depositionmethod is a technique in which an aerosol is formed by dispersing powderin a gas through, for example, vibration; the thus-formed aerosol isconveyed to a deposition chamber which has been evacuated to apredetermined level; and the aerosol is sprayed through a nozzle onto apredetermined substrate, to thereby form a dielectric layer. Powder jetdeposition method is a technique in which powder is conveyed by means ofhigh-pressure gas, and is sprayed, at high speed, through a nozzle to asubstrate provided in air, to thereby form a dielectric layer.

Typical examples of the aforementioned dielectric device include apiezoelectric actuator and an electron emitter.

A piezoelectric actuator is configured such that when a predeterminedelectric field is applied to a dielectric layer through application of apredetermined voltage between a lower electrode layer and an upperelectrode, the dielectric layer can be deformed. For example, JapanesePatent Application Laid-Open (Kokai) No. 2002-217465 discloses aunimorph-type piezoelectric actuator including a substrate and adielectric layer bonded thereonto, the actuator being configured suchthat the substrate can be bent or deformed through expansion andcontraction of the dielectric layer through the transverse piezoelectriceffect.

Meanwhile, an electron emitter is configured such that it can besuitably employed as an electron beam source in a variety of apparatusesthat utilize electron beams, including a display (e.g., a field emissiondisplay (FED)), an electron beam irradiation apparatus, a light source,an electronic-component-manufacturing apparatus, and an electroniccircuit component.

Such an electron emitter (i.e., a type of the aforementioned dielectricdevice) includes an emitter section which is provided in areduced-pressure atmosphere having a predetermined vacuum level. Theemitter section, which includes a dielectric layer, is configured suchthat it can emit electrons into the reduced-pressure atmosphere throughapplication of a predetermined driving electric field between a lowerelectrode layer and an upper electrode. Conventionally known electronemitters of this type include those disclosed in Japanese PatentApplication Laid-Open (Kokai) No. 2005-183361 and the specification ofU.S. Patent Application Publication No. 2006/0012279.

Such an electron emitter (i.e., a type of the aforementioned dielectricdevice) is operated as follows. Firstly, in the first stage, voltage isapplied between an upper electrode and a lower electrode layer so thatthe upper electrode is higher in electric potential. An electric fieldgenerated by the applied voltage brings the emitter section of theelectron emitter into a predetermined polarization state.

Subsequently, in the second stage, voltage is applied between the upperelectrode and the lower electrode layer so that the upper electrode islower in electric potential. Through this voltage application, thepolarization of the emitter section is inverted, and electrons areaccumulated on an electron emission region.

Subsequently, in the third stage, voltage is again applied so that theupper electrode is higher in electric potential. Through this voltageapplication, the polarization of the emitter section is re-inverted.With this polarization inversion, the electrons accumulated on theelectron emission region are emitted from the emitter section by meansof electrostatic repulsion between the electrons and dipoles, and thethus-emitted electrons fly in the aforementioned reduced-pressureatmosphere.

SUMMARY OF THE INVENTION

The present invention provides a dielectric device exhibiting higherperformance as compared with a conventional dielectric device. Forexample, when the dielectric device of the present invention is in theform of a piezoelectric actuator, the piezoelectric actuator provides anincreased amount of deformation through application of a predeterminedvoltage. Alternatively, when the dielectric device of the presentinvention is in the form of an electron emitter, the electron emitterprovides an increased quantity of electrons to be emitted throughapplication of a predetermined voltage, and exhibits enhanced electronemission efficiency.

In one aspect of the present invention, the dielectric device includes adielectric layer. The dielectric layer may be formed on a predeterminedsubstrate. The dielectric layer may be bonded directly onto thesubstrate, or may be bonded, via a predetermined base electrode layer,to the substrate.

A characteristic feature of the dielectric device resides in that thedielectric layer is formed by spraying a powdery dielectric material anda fine-particulate metal toward the substrate to thereby yield a layerwhich is to become the dielectric layer (hereinafter the layer to becomethe dielectric layer may be referred to a “film layer”), and bythermally treating the film layer for causing migration of the metal inthe film layer so that the metal content by volume of the dielectriclayer is 10% or less, and the porosity of the dielectric layer fallswithin a range of 2% to 20%.

With this configuration, the film layer has a dense structure in whichthe fine-particulate metal is dispersed in the matrix of the dielectricmaterial, and exhibits high dielectric constant. As used herein, theexpression “the fine-particulate metal is dispersed in the matrix of thedielectric material” refers to the state where finely divided particlesof the metal are present discretely in the matrix. The fine metalparticles are not necessarily dispersed uniformly in the film layer. Forexample, the fine metal particles may be non-uniformly dispersed withrespect to a thickness direction of the film layer.

Thermal treatment of the thus-formed dense film layer reduces latticedefect, lattice strain, etc. in the dielectric layer, and results inprogress of grain growth and improvement of properties of the layer.Migration of the metal in the film layer associated with the thermaltreatment releases compressive stress in the film layer, which stresswould otherwise occur during film formation. This greatly improvesproperties (e.g., piezoelectric property and polarization inversionproperty) of the dielectric layer.

Thus, the dielectric device provided by the present invention exhibitshigher performance as compared with a conventional dielectric device.For example, according to the present invention, in the aforementionedpiezoelectric actuator, the amount of deformation through application ofa predetermined voltage can be increased. Alternatively, according tothe present invention, in the aforementioned electron emitter (i.e., atype of the dielectric device), the quantity of electrons to be emittedthrough application of a predetermined voltage can be increased, andelectron emission efficiency can be enhanced.

In contrast, when the metal content by volume of the dielectric layerexceeds 10%, the breakdown voltage of the entirety of the layer islowered, whereas when the porosity of the layer exceeds 20%, thebreakdown voltage of the layer is lowered, and properties (e.g.,polarization inversion property and piezoelectric property) of the layerare impaired. Meanwhile, when the porosity of the dielectric layer isless than 2%, properties (e.g., polarization inversion property andpiezoelectric property) of the layer fail to be improved.

In another aspect of the present invention, the dielectric deviceincludes a substrate, a base electrode, and a dielectric layer. The baseelectrode is formed of a conductor film provided on the substrate. Thedielectric layer is formed so as to contain a metal, and is provided onthe base electrode.

A drive electrode may be formed on the dielectric layer. The driveelectrode, which is formed of a conductor film, may be provided suchthat the dielectric layer is sandwiched between the drive electrode andthe base electrode. That is, the drive electrode may be formed on anouter surface of the dielectric layer. The outer surface is opposite thesurface of the dielectric layer which faces the base electrode. Thedielectric device may be configured so that a predetermined drivingelectric field is applied to the dielectric layer through application ofa drive voltage having a predetermined waveform between the baseelectrode and the drive electrode.

A characteristic feature of the dielectric device resides in that thedielectric layer is formed such that the metal content of the layerdiffers from portion to portion in a thickness direction of the layer.The dielectric layer can be formed by spraying a powdery dielectricmaterial and a fine-particulate metal toward a predetermined substrateto thereby yield a film layer, and by thermally treating the film layerfor causing migration of the metal in the film layer.

With this configuration, the metal contained in the dielectric layer canenhance the dielectric constant of the layer. That is, the metalcontained in the dielectric layer can suppress reduction of thedielectric constant of the layer attributed to defects in the layer.

In the dielectric layer, the metal content differs from portion toportion in a thickness direction of the layer. Therefore, theaforementioned properties (e.g., piezoelectric property and polarizationinversion property) of the dielectric layer can be varied in thethickness direction.

For example, the metal content of a portion of the dielectric layer inthe vicinity of the outer surface of the layer (hereinafter the portionmay be referred to as an “outer-surface-adjacent portion”) can beregulated to be higher than that of a portion (other than theouter-surface-adjacent portion) of the dielectric layer (hereinafter theportion other than the outer-surface-adjacent portion may be referred toas an “outer-surface-distant portion”). Specifically, for example, themetal content of the outer-surface-adjacent portion can be regulated tobe ¼ or less that of the outer-surface-distant portion. Alternatively,for example, the metal content of the outer-surface-adjacent portion canbe regulated to be almost zero (i.e., the metal is not specified throughimage analysis by use of an electron micrograph, etc., or, for example,the metal content as detected by means of a predetermined analyzer istwice or less the detection limit of the analyzer).

With this configuration, the dielectric layer can be formed such thatthe dielectric constant of the outer-surface-distant portion is higherthan that of the outer-surface-adjacent portion. In this case, whenvoltage is applied in the aforementioned thickness direction, electricfield concentration may occur at the outer-surface-adjacent portion.Therefore, polarization inversion or a similar operation may occur atthe outer-surface-adjacent portion to a more remarkable extent.

The thus-formed dielectric layer having the aforementioned configurationis more suitable for application or performance of the dielectricdevice. Thus, the present invention can provide a dielectric deviceexhibiting higher performance as compared with a conventional dielectricdevice.

The dielectric layer may be formed such that the metal content of afirst portion; i.e., a portion located between the center of the layerin a thickness direction towards the base electrode, is higher than thatof a second portion; i.e., a portion of the layer other than the firstportion. In this case, preferably, the dielectric layer is formed suchthat the metal content by volume of the first portion is 2% to 20%, andthe metal content by volume of the second portion is 5% or less.

With this configuration, the dielectric constant of the first portion,which is located near the base electrode, can become higher than that ofthe second portion, which is located away from the base electrode. Inthis case, when voltage is applied in the aforementioned thicknessdirection, electric field concentration occurs at the second portion.Therefore, polarization inversion, deformation through the conversepiezoelectric effect, or a similar operation occurs at the secondportion to a more remarkable extent.

In contrast, when the metal content of the first portion exceeds 20%,dielectric properties of the first portion are lost, whereas when themetal content of the first portion is less than 2%, properties (e.g.,polarization inversion property and piezoelectric property) of thedielectric layer fail to be improved. Meanwhile, when the metal contentof the second portion exceeds 5%, properties (e.g., polarizationinversion property and piezoelectric property) of the dielectric layerare impaired.

The dielectric device of the present invention, which has theaforementioned configuration, can be produced through the followingprocedure. Firstly, a powdery dielectric material and a fine-particulatemetal are sprayed toward a substrate, to thereby form a film layer inwhich the metal is dispersed in the matrix of the dielectric material(film layer formation step). Subsequently, the film layer formed in thefilm layer formation step is thermally treated for causing migration ofthe metal in the film layer, to thereby yield a dielectric layer(thermal treatment step). The thermal treatment step causes thedielectric layer to contain the metal in a predetermined state.

Specifically, the aforementioned film layer formation step forms, athigh efficiency, the film layer having a dense structure and a highdielectric constant, in which the metal is dispersed in the matrix ofthe dielectric material. In the aforementioned thermal treatment step,thermal treatment of the thus-formed dense film layer, and migration ofthe metal in the film layer associated with the thermal treatment cansuppress impairment of properties (e.g., dielectric constant) of thedielectric layer, which impairment is due to residual stress or thepresence of defects, etc. in the dielectric layer. Therefore, thethus-formed dielectric layer exhibits greatly improved piezoelectric andpolarization inversion properties.

As described above, according to the present invention, a dielectricdevice exhibiting higher performance as compared with a conventionaldielectric device can be produced through a very simple productionprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and many of the attendant advantages ofthe present invention will be readily appreciated as the same becomesbetter understood with reference to the following detailed descriptionof the preferred embodiments when considered in connection with theaccompanying drawings, in which:

FIG. 1 is an enlarged cross-sectional view showing a dielectric deviceaccording to an embodiment of the present invention;

FIG. 2 is a flowchart schematically showing the production method forthe dielectric device according to the embodiment;

FIG. 3 schematically shows the configuration of an aerosol depositionapparatus employed in the film layer formation step shown in FIG. 2;

FIG. 4 is a cross-sectional view schematically showing a display towhich the dielectric device according to the embodiment is applied;

FIG. 5 is an enlarged cross-sectional view showing essential portions ofthe electron emitter of FIG. 4;

FIG. 6 shows an equivalent circuit of the electron emitter of FIG. 4;

FIG. 7 shows an equivalent circuit of the electron emitter of FIG. 4;

FIG. 8 is a diagram showing the waveform of a drive voltage applied tothe electron emitter of FIG. 4;

FIGS. 9A to 9C show the state of operation of the electron emitter ofFIG. 4 in the case where the drive voltage shown in FIG. 8 is applied tothe electron emitter;

FIGS. 10A to 10C show the state of operation of the electron emitter ofFIG. 4 in the case where the drive voltage shown in FIG. 8 is applied tothe electron emitter;

FIGS. 11A and 11B are cross-sectional views schematically showing theconfiguration of a piezoelectric actuator, which is a dielectric deviceaccording to the embodiment;

FIGS. 12A and 12B are enlarged cross-sectional views showing essentialportions of the piezoelectric actuator of FIGS. 11A and 11B;

FIG. 13 shows the Q-V hysteresis of a dielectric material constitutingthe emitter layer shown in FIG. 4; and

FIG. 14 schematically shows the configuration of an aerosol depositionapparatus employed in the film layer formation step shown in FIG. 2, theapparatus being employed in Examples 3 and 4.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment and Examples of the dielectric device of thepresent invention will next be described with reference to the drawingsand tables. The material and structure of components of the dielectricdevice of the present invention will be described with reference to onetypical embodiment, for the sake of readily understandable andconsistent illustration. Modifications of the material and structure ofthe components of the dielectric device according to the embodiment willbe collectively described after description of the configuration,operation, and effect of the dielectric device according to theembodiment.

Schematic Description of Dielectric Device

FIG. 1 is an enlarged cross-sectional view of a dielectric device 10according to the present embodiment. As shown in FIG. 1, the dielectricdevice 10 includes a substrate 11, a lower electrode 12, a dielectriclayer 13, and an upper electrode 14.

The substrate 11 is formed of a glass or ceramic plate material. Theceramic material employed for forming the substrate 11 is preferably aceramic material containing at least one species selected from the groupconsisting of aluminum oxide, magnesium oxide, mullite, aluminumnitride, silicon nitride, and stabilized zirconium oxide, from theviewpoints of heat resistance, chemical stability, and insulatingproperty. Particularly preferably, the substrate 11 is formed ofstabilized zirconium oxide, from the viewpoints of high mechanicalstrength and excellent toughness.

As used herein, the term “stabilized zirconium oxide” refers tozirconium oxide in which crystal phase transition is suppressed throughaddition of a stabilizer. The stabilized zirconium oxide encompassespartially stabilized zirconium oxide. Examples of the stabilizedzirconium oxide include zirconium oxide containing a stabilizer (e.g.,calcium oxide, magnesium oxide, yttrium oxide, scandium oxide, ytterbiumoxide, cerium oxide, or an oxide of a rare earth metal) in an amount of1 to 30 mol %. From the viewpoint of enhancement of mechanical strength,zirconium oxide containing yttrium oxide as a stabilizer is particularlypreferred. In this case, the yttrium oxide content is preferably 1.5 to6 mol %, more preferably 2 to 4 mol %. Zirconium oxide containing, inaddition to yttrium oxide, aluminum oxide in an amount of 0.1 to 5 mol %is more preferred.

The stabilized zirconium oxide may have, for example, a cubic-monocliniccrystal phase, a tetragonal-monoclinic crystal phase, or acubic-tetragonal-monoclinic crystal phase. From the viewpoints ofstrength, toughness, and durability, the stabilized zirconium oxidepreferably has, as a primary crystal phase, a tetragonal crystal phaseor a tetragonal-cubic crystal phase.

The lower electrode 12 is formed on the substrate 11. The lowerelectrode 12, which is an electrode member constituting a base electrodein the present invention, is formed of a conductor film. The conductorfilm constituting the lower electrode 12 may be formed of, for example,a metallic material, a cermet, a carbon material, or an oxide material.These materials may be employed singly or in combination.

Examples of the metallic material which may be employed include gold,silver, platinum, iridium, palladium, rhodium, molybdenum, and tungsten.The metallic material may be any of such metallic elements or an alloyformed of such metallic elements. Examples of preferably employed alloysinclude silver-palladium alloy, silver-platinum alloy, andplatinum-palladium alloy. Examples of preferably employed cermetsinclude a cermet material containing platinum and a ceramic material.

Examples of the carbon material which may be employed include graphite,diamond thin film, diamond-like carbon, and carbon nanotube.

Examples of the oxide material which may be employed include rutheniumoxide, iridium oxide, strontium ruthenate, La_(1-x)Sr_(x)CoO₃ (e.g.,x=0.3 or 0.5), La_(1-x)Ca_(x)MnO₃, La_(1-x)Ca_(x)Mn_(1-y)CO_(y)O₃ (e.g.,x=0.2, y=0.05), and indium tin oxide (ITO).

The dielectric layer 13 is provided such that the lower electrode 12 issandwiched between the layer 13 and the substrate 11. The dielectriclayer 13 has an upper surface 13 a, which is opposite a lower surface 13b of the layer 13 facing the lower electrode 12. The upper electrode 14,which is formed of a conductor film, is formed on the upper surface 13a. The conductor film constituting the upper electrode 14 may be formedof any of the aforementioned materials, such as a metallic material, acermet, a carbon material, or an oxide material.

The dielectric layer 13 is formed of a metal-containing dielectricmaterial. The dielectric material which primarily constitutes thedielectric layer 13 is preferably a dielectric material having arelatively high specific dielectric constant (e.g., 1,000 or more).Examples of such a dielectric material include barium titanate, leadzirconate, lead magnesium niobate, lead nickel niobate, lead zincniobate, lead manganese niobate, lead magnesium tantalate, lead nickeltantalate, lead antimony stannate, lead titanate, lead magnesiumtungstate, and lead cobalt niobate.

The dielectric layer 13 may be formed of a ceramic material containingan arbitrary combination of such dielectric materials. The dielectriclayer 13 may also be formed of a ceramic material containing, as aprimary component, such a dielectric material in an amount of 50 wt. %or more. The dielectric layer 13 may also be formed of a materialcontaining any of the aforementioned dielectric materials and ceramicmaterials, and further containing a compound appropriately selected fromamong, for example, oxides of lanthanum, calcium, strontium, molybdenum,tungsten, barium, niobium, zinc, nickel, manganese, etc. (these may beappropriately employed in combination).

The material employed for forming the dielectric layer 13 is preferably,for example, a binary dielectric material containing lead magnesiumniobate (PMN) and lead titanate (PT); i.e., nPMN-mPT (n, m: ratio bymole), which has, through an increase in PMN ratio, a lowered Curiepoint and an increased specific dielectric constant as measured at roomtemperature. In the nPMN-mPT dielectric material, particularlypreferably, n is 0.85 to 1.0 and m is 1.0−n. In this case, the specificdielectric constant is 3,000 or more. For example, when n is 0.91 and mis 0.09, the specific dielectric constant as measured at roomtemperature is 15,000, whereas when n is 0.95 and m is 0.05, thespecific dielectric constant as measured at room temperature is 20,000.

Among ternary materials containing lead magnesium niobate (PMN), leadtitanate (PT), and lead zirconate (PZ), a material in which theproportion by mole of PMN is high, or a material having a composition inthe vicinity of the morphotropic phase boundary (MPB) between tetragonaland pseudocubic phases or between tetragonal and rhombohedral phases ispreferably employed as the aforementioned dielectric material by virtueof its high specific dielectric constant. Particularly preferred is amaterial (PMN:PT:PZ=0.375:0.375:0.25) having a specific dielectricconstant of 5,500, or a material (PMN:PT:PZ=0.5:0.375:0.125) having aspecific dielectric constant of 4,500.

Examples of the metal which may be contained in the dielectric layer 13include silver, copper, gold, and platinum. Particularly, gold orsilver, which has a low melting point and is not easily oxidized, ispreferably employed. Such a metal is dispersed in the matrix of theaforementioned dielectric material. As used herein, the expression “sucha metal is dispersed in the matrix of the aforementioned dielectricmaterial” refers to the state where finely divided particles of themetal are present discretely in the matrix to such an extent that themetal particles do not impair properties of the dielectric layer 13serving as a dielectric thin film, including piezoelectric effect,converse piezoelectric effect, and polarization inversion.

The dielectric layer 13 is formed under predetermined film formationconditions such that the layer 13 has a predetermined porosity andcontains the aforementioned metal in a predetermined state.Specifically, the dielectric layer 13 is formed such that the porosityfalls within a range of 2% to 20%. The dielectric layer 13 is formedsuch that the metal content by volume of the layer 13 is 10% or less onthe basis of the entirety of the layer 13.

The dielectric layer 13 is formed such that the metal content of thelayer differs from portion to portion in a thickness direction.Specifically, the dielectric layer 13 is formed such that the metalcontent of a portion 13 c in the vicinity of the lower electrode(hereinafter such a portion may be referred to as a“lower-electrode-adjacent portion”) differs from that of a portion 13 din the vicinity of the upper surface (hereinafter such a portion may bereferred to as an “upper-surface-adjacent portion”). Thelower-electrode-adjacent portion 13c is located between the center (in athickness direction) of the layer 13 and towards the lower electrode 12.The upper-surface-adjacent portion 13 d is located between the center(in a thickness direction) of the layer 13 and towards the upper surface13 a.

The dielectric device 10 is configured such that the device can bedriven when a predetermined driving electric field is applied to thedielectric layer 13 through application of a drive voltage having apredetermined waveform between the lower electrode 12 and the upperelectrode 14.

Dielectric Device Production Method

Next will be described a production method for the dielectric device 10according to the present embodiment with appropriate reference to theaforementioned reference numerals of FIG. 1 for illustrating componentsof the dielectric device 10.

FIG. 2 is a flowchart schematically showing the production method. Asshown in FIG. 2, the production method includes a lower electrodeformation step S1 (“S” is an abbreviation of “step,” the same shallapply hereinafter); a film layer formation step S2; a thermal treatmentstep S3; and a lower electrode formation step S4.

Firstly, a lower electrode 12 is formed on a substrate 11 through thelower electrode formation step S1. The lower electrode formation step S1may employ a generally employed film formation technique, such as athick film formation technique (e.g., spin coating, screen printing,spraying, coating, dipping, application, or electrophoresis) or a thinfilm formation technique (e.g., sputtering, the ion beam process, vacuumdeposition, ion plating, chemical vapor deposition (CVD), or plating).

Subsequently, a film layer containing the aforementioned dielectricmaterial and metal is formed on the lower electrode 12 through the filmlayer formation step S2. In the film layer formation step S2, powder ofthe aforementioned dielectric material and finely divided particles ofthe aforementioned metal are sprayed toward the substrate 11, to therebyform a film layer in which the metal is dispersed in the matrix of thedielectric material. The film layer formation step S2 preferably employsso-called aerosol deposition method. Aerosol deposition method is atechnique in which powder of the aforementioned dielectric material andfinely divided particles of the aforementioned metal are dispersed in agas through vibration or the like to form an aerosol, and subsequentlythe thus-formed aerosol is conveyed to a deposition chamber which hasbeen evacuated to a predetermined level, followed by spraying of theaerosol through a nozzle onto the substrate 11, to thereby form theaforementioned film layer.

Subsequently, the film layer is thermally treated through the thermaltreatment step S3. This thermal treatment causes migration of theaforementioned metal in the film layer. This metal migration forms adielectric layer 13 containing the metal in a predetermined state.

Subsequently, an upper electrode 14 is formed on the dielectric layer 13through the upper electrode formation step S4. The upper electrodeformation step S4 can be carried out in a manner similar to that of thelower electrode formation step S1.

Specific Example Of Dielectric Layer Formation Through AerosolDeposition Method

FIG. 3 schematically shows the configuration of an aerosol depositionapparatus 60 employed in the film layer formation step S2 shown in FIG.2. The aerosol deposition apparatus 60 includes the deposition chamber70 and an aerosol supply unit 80.

The deposition chamber 70 includes a vacuum container 71, an XYZθ stage72, a nozzle 73, and a vacuum pump 74. The vacuum container 71 isconfigured such that the vacuum of its interior can be maintained at apredetermined level. The XYZθ stage 72 is configured such that it cansupport the substrate 11 thereon within the vacuum container 71 and canmove the substrate 11 in an arbitrary direction. The nozzle 73 isfixated in the vacuum container 71. The nozzle 73 is configured suchthat an aerosol can be sprayed onto the substrate 11 supported on theXYZθ stage 72. The vacuum pump 74 is configured such that it canevacuate air from the vacuum container 71 so as to maintain the vacuumof the container 71 at the aforementioned predetermined level.

The aerosol supply unit 80 is configured such that raw material powder81 sprayed through the nozzle 73 onto the substrate 11 can be suppliedto the nozzle 73. The raw material powder 81 is a mixture of theaforementioned dielectric material powder and the aforementioned finemetal particles.

The aerosol supply unit 80 includes an aerosol chamber 82, a compressedgas supply source 83, a compressed gas supply tube 84, a vibrationstirring section 85, an aerosol supply tube 86, and a control valve 87.

The aerosol chamber 82 is configured as a container capable of storingthe raw material powder 81. The compressed gas supply source 83 isconfigured such that it can store a carrier gas which is mixed with theraw material powder 81 in the aerosol chamber 82 to form an aerosol. Thecarrier gas to be employed may be compressed air, a rare gas (e.g.,helium or argon), or an inert gas (e.g., nitrogen gas). The compressedgas supply tube 84 is configured such that the aforementioned carriergas can be supplied from the compressed gas supply source 83 to theaerosol chamber 82. The vibration stirring section 85 is configured suchthat it vibrates the aerosol chamber 82 so as to enable an aerosol to beformed through mixing of the raw material powder 81 with the carrier gasin the aerosol chamber 82. The aerosol supply tube 86 is configured suchthat the aerosol formed in the aerosol chamber 82 can be supplied to thenozzle 73. The control valve 87 is configured such that it can controlthe flow rate of the aerosol passing through the aerosol supply tube 86to thereby regulate the amount of the aerosol sprayed through the nozzle73 onto the substrate 11.

The aerosol deposition apparatus 60 having the above-describedconfiguration is operated as follows.

The raw material powder 81 is vigorously mixed with the aforementionedcarrier gas in the aerosol chamber 82 through vibration generated bymeans of the vibration stirring section 85. This mixing forms an aerosolin the aerosol chamber 82. The thus-formed aerosol behaves as a fluid.Therefore, when the control valve 87 is in an open state, the aerosolcan flow toward the vacuum container 71 by means of the difference inpressure between the aerosol chamber 82 and the vacuum container 71, andthe aerosol can be sprayed through the nozzle 73 onto the substrate 11at high speed. When the control valve 87 is opened, and the aerosolcontaining the raw material powder 81 is sprayed onto the substrate 11,the aforementioned film layer (which is to become the dielectric layer13) is formed on the substrate 11 (accurately, on the lower electrode 12shown in FIG. 1).

According to the above-described film layer formation step S2, in whichthe aforementioned dielectric material powder and fine metal particlesare sprayed onto the substrate 11 through aerosol deposition, the powderand finely divided particles undergo crushing and plastic deformation,and are deposited onto the substrate. Therefore, a dense film layer isformed at room temperature.

When the aforementioned dielectric material powder and fine metalparticles are simultaneously sprayed onto the substrate 11, the finemetal particles, which have ductility, serve as a bonding agent.Therefore, film formation on the substrate 11 is facilitated.

Schematic Description of FED

FIG. 4 is a cross-sectional view schematically showing a display 100,which is an FED to which the dielectric device according to the presentembodiment is applied.

As shown in FIG. 4, the display 100 includes a light-emitting panel 101.The light-emitting panel 101 includes a transparent plate 101 a, acollector electrode 101 b, and a phosphor layer 101 c.

The transparent plate 101 a is formed of a glass plate or an acrylicplate. The collector electrode 101 b is formed on the surface on thelower side (as viewed in FIG. 4) of the transparent plate 101 a. Thecollector electrode 101 b is formed of a transparent electrode such asan indium tin oxide (ITO) thin film.

The phosphor layer 101 c is formed on the lower surface of the collectorelectrode 101 b. The phosphor layer 101 c is configured such that whenelectrons flying toward the collector electrode 101 b, which isconnected to a bias voltage source 102 via a predetermined resistor,collide with the phosphor layer 101 c, fluorescence can be emitted. Thebias voltage source 102 is configured to apply a predetermined collectorvoltage Vc between the ground and the collector electrode 101 b.

As shown in FIG. 4, an electron-emitting device 110 is provided belowthe light-emitting panel 101. The electron-emitting device 110 iselectrically connected to a pulse generator 110 a. The electron-emittingdevice 110 is configured such that when a drive voltage Va is appliedthereto by means of the pulse generator 110 a, electrons are emittedtoward the light-emitting panel 101 (the collector electrode 101 b andthe phosphor layer 101 c).

A predetermined space is provided between the electron-emitting device110 and the light-emitting panel 101 (phosphor layer 101 c). The spacebetween the electron-emitting device 110 and the phosphor layer 101 c isa reduced-pressure atmosphere having a predetermined vacuum level of,for example, 10² to 11⁻⁶ Pa (more preferably 10⁻³ to 10⁻⁵ Pa).

The display 100 is configured such that electrons are emitted, to thereduced-pressure atmosphere, from the electron-emitting device 110through application of the drive voltage Va to the device 110 by meansof the pulse generator 110 a, and that, by means of an electric fieldgenerated through application of the collector voltage Vc, thethus-emitted electrons fly toward the collector electrode 101 b andcollide with the phosphor layer 101 c, whereby fluorescence is emitted.

Configuration of Electron-Emitting Device

The electron-emitting device 110 is configured so as to have a thin flatplate shape. The electron-emitting device 110 includes a number oftwo-dimensionally arranged electron emitters 120 according to thepresent embodiment. Each of the electron emitters 120 includes asubstrate 121, a lower electrode 122, an emitter layer 123, and an upperelectrode 124.

The substrate 121 is formed of a heat-resistant glass thin plate or aceramic thin plate. The lower electrode 122 is formed on the substrate121. The lower electrode 122 is electrically connected to theaforementioned pulse generator 110 a. The emitter layer 123, whichconstitutes a dielectric layer according to the present invention, isprovided on the lower electrode 122.

The emitter layer 123 is formed to have a thickness of 1 to 300 μm (morepreferably about 5 to about 100 μm). Microscopic concavities andconvexities due to, for example, crystal grain boundaries are formed onan upper surface 123 a of the emitter layer 123. Specifically, numerousconcavities 123 a 1 are formed on the upper surface 123 a. The uppersurface 123 a is formed so as to have a surface roughness Ra (centerlineaverage roughness, unit: μm) of 0.005 or more and 3.0 or less. Theemitter layer 123 is formed on the lower electrode 122 such that a lowersurface 123 b of the layer 123, which is opposite the upper surface 123a, is in contact with the lower electrode 122.

In the present embodiment, as described above, a portion of the emitterlayer 123 on the side of the lower surface 123 b (i.e., alower-electrode-adjacent portion 123 c) and a portion of the emitterlayer 123 on the side of the upper surface 123 a (i.e., anupper-surface-adjacent portion 123 d) are formed under predeterminedfilm formation conditions such that each of the portions 123 c and 123 dhas a predetermined porosity and contains the aforementioned metal in apredetermined state.

The upper electrode 124 is provided on the upper surface 123 a of theemitter layer 123. The upper electrode 124 is formed so as to have athickness of about 0.1 to about 20 μm. The upper electrode 124 iselectrically connected to the aforementioned pulse generator 110 a.

The upper electrode 124 has a plurality of openings 124 a. The openings124 a are formed such that the upper surface 123 a of the emitter layer123 is exposed to the outside of the electron-emitting device 110 (i.e.,the aforementioned reduced-pressure atmosphere; the same shall applyhereinafter). The upper surface 123 a of the emitter layer 123 isexposed to the outside of the electron-emitting device 110 also atperipheral edge portions 124 b of the upper electrode 124. A portion ofthe emitter layer 123 exposed to the outside of the electron-emittingdevice 110 constitutes an emitter section 125, which serves as a mainsection for electron emission.

As described below, the electron emitter 120 is configured such thatelectrons supplied from the upper electrode 124 are accumulated on theemitter section 125, and the thus-accumulated electrons are emittedtoward the outside of the electron-emitting device 110 (i.e., toward thephosphor layer 101 c).

Detailed Description of Electron Emitter

FIG. 5 is an enlarged cross-sectional view showing essential portions ofthe electron emitter 120 of FIG. 4. In the case shown in FIG. 5 (or FIG.4), the concavities 123 a 1 and the openings 124 a are formed inone-to-one correspondence. However, in some cases, a plurality ofconcavities 123 a 1 may be formed in a single opening 124 a, or noconcavities 123 a 1 may be formed in an opening 124 a.

As shown in FIG. 5, in the upper electrode 124, a peripheral portion126, which is a portion in the vicinity of the opening 124 a, isprovided so as to overhang the emitter section 125 (hereinafter theportion may be referred to as an “overhanging portion”). Specifically,the overhanging portion 126 is formed such that a lower surface 126 aand a tip end 126 b of the overhanging portion 126 are apart from theupper surface 123 a of the emitter layer 123 corresponding to theemitter section 125. The overhanging portion 126 is also formed atpositions corresponding to the peripheral edge portions 124 b (see FIG.4) of the upper electrode 124.

A triple junction 126 c is formed at a position at which the overhangingportion 126 is in contact with the upper surface 123 a of the emitterlayer 123; i.e., at a position at which the emitter layer 123 is incontact with the upper electrode 124 and the aforementionedreduced-pressure atmosphere.

The triple junction 126 c is a site (electric field concentration point)at which lines of electric force concentrate (where electric fieldconcentration occurs) when, as shown in FIG. 4, a drive voltage Va isapplied between the lower electrode 122 and the upper electrode 124. Asused herein, the expression “site at which lines of electric forceconcentrate” refers to a site at which lines of electric force that aregenerated from the lower electrode 122 at even intervals concentrate,when the electric force lines are drawn under the assumption that thelower electrode 122, the emitter layer 123, and the upper electrode 124are flat plates each having a cross section extending infinitely. Thestate of the concentration of lines of electric force (i.e., the stateof electric field concentration) can be readily observed throughsimulation by means of numerical analysis employing the finite-elementmethod.

As shown in FIG. 5, a gap 127 is formed between the lower surface 126 aand tip end 126 b of the overhanging portion 126 and the upper surface123 a (emitter section 125) of the emitter layer 123. The gap 127 isformed such that the maximum gap d satisfies the following relation: 0μm<d≦10 μm, and the angle θ between the lower surface 126 a and thesurface of the emitter section 125 satisfies the following relation:1°≦θ≦60°.

The tip end 126 b of the overhanging portion 126 has such a shape as toserve as the aforementioned electric field concentration point.Specifically, the overhanging portion 126 has such a cross-sectionalshape as to be acutely pointed toward the tip end 126 b of the portion126; i.e., the thickness gradually decreases.

The openings 124 a may be formed to assume a variety of shapes as viewedin plane (as viewed from above in FIG. 5), including a circular shape,an elliptical shape, a polygonal shape, and an irregular shape. Theopenings 124 a are formed such that the average of diameters of theopenings 124 a as viewed in plane is 0.1 μm or more and 20 μm or less.The reason for this is described below. As used herein, the expression“the average of diameters of the openings 124 a” refers to thenumber-based average of diameters of circles having areas identical tothose of the openings 124 a.

As shown in FIG. 5, regions of the emitter layer 123 where polarizationis inverted in accordance with application of the aforementioned drivevoltage (drive voltage Va shown in FIG. 4) are first regions 128 andsecond regions 129. The first regions 128 correspond to regions facingthe upper electrode 124. The second regions 129 correspond to regions ofthe openings 124 a that extend from the tip ends 126 b of theoverhanging portions 126 toward the centers of the openings 124 a. Therange of the second regions 129 varies depending on the level of thedrive voltage Va and the degree of electric field concentration in thevicinity of the second regions 129.

When the average diameter of the openings 124 a falls within theabove-described range (i.e., 0.1 μm or more and 20 μm or less), asufficient quantity of electrons are emitted through the openings 124 a,and high electron emission efficiency is secured.

When the average diameter of the openings 124 a is less than 0.1 μm, thearea of the second regions 129 decreases. The second regions 129constitute primary regions of the emitter section 125 which temporarilyaccumulates electrons supplied from the upper electrode 124 and thenemits the electrons. Therefore, a decrease in area of the second regions129 results in reduction of the quantity of electrons to be emitted. Incontrast, when the average diameter of the openings 124 a exceeds 20 μm,the ratio of the second regions 129 to the entirety of the emittersection 125 (occupancy of the second regions) decreases, resulting inreduction of electron emission efficiency.

Equivalent Circuit of Electron Emitter

FIGS. 6 and 7 show equivalent circuits of the electron emitter 120 ofFIG. 4.

Most briefly, the configuration of the electron emitter 120 according tothe present embodiment can be approximated to an equivalent circuit asshown in FIG. 6. “C1” of FIG. 6 is a capacitor formed by sandwiching theemitter layer 123 between the lower electrode 122 and the upperelectrode 124. “Ca” of FIG. 6 is a capacitor formed by any of the gaps127 (see FIG. 5). “C2” of FIG. 6 is a capacitor formed of an aggregateof a plurality of capacitors Ca, which are connected in parallel. Thecapacitor C1 associated with the emitter layer 123 is connected inseries to the capacitor C2 associated with the gaps 127 (see FIG. 5).

However, the equivalent circuit, in which the capacitor C1 associatedwith emitter layer 123 is connected in series to the capacitor C2 formedof the aggregate of the capacitors Ca is not practical. In practice,conceivably, the percentage of a portion of the capacitor C1 associatedwith the emitter layer 123 that is connected in series to the capacitorC2 formed of the capacitor aggregate varies with, for example, thenumber and area of the openings 124 a (see FIG. 5) formed in the upperelectrode 124.

Capacitance will now be calculated under the assumption that, forexample, 25% of the capacitor C1 associated with the emitter layer 123is connected in series to the capacitor C2 as shown in FIG. 7.

Conditions of the calculation are as follows: the gaps 127 are in avacuum (i.e., specific dielectric constant ∈_(r)=1); the maximum gap dof the gaps 127 is 0.1 μm; the area S of a region corresponding to asingle gap 127 is 1 μm×1 μm; the number of the gaps 127 is 10,000; thespecific dielectric constant of the emitter layer 123 is 2,000; thethickness of the emitter layer 123 is 20 μm; and the facing area betweenthe lower electrode 122 and the upper electrode 124 is 200 μm×200 μm.

Under the above-described conditions, the capacitance of the capacitorC1 is 35.4 pF, and the capacitance of the capacitor C2 is 0.885 pF. Theoverall capacitance between the upper electrode 124 and the lowerelectrode 122 is 27.5 pF, which is lower than the capacitance of thecapacitor C1 associated with the emitter layer 123 (i.e., 35.4 pF);i.e., the overall capacitance is 78% the capacitance of the capacitorC1.

As described above, the overall capacitance of the capacitor C2 formedof the aggregate of the capacitors Ca associated with the gaps 127 (seeFIG. 5) is considerably lower than the capacitance of the capacitor C1(associated with the emitter layer 123) which is connected in series tothe capacitor C2. Therefore, when the drive voltage Va is applied tothis series circuit, most of the voltage Va is applied to the capacitorsCa (C2), whose capacitance is lower than that of the capacitor C1. Inother words, most of the drive voltage Va is applied to the gaps 127(see FIG. 5). This attains an increase in output of the electronemitter.

As described above, the capacitor C1 associated with the emitter layer123 is connected in series to the capacitor C2 formed of the aggregateof the capacitors Ca associated with the gaps 127 (see FIG. 5).Therefore, the overall capacitance of this series circuit is lower thanthe capacitance of the capacitor C1 associated with the emitter layer123. Therefore, the electron emitter exhibits a preferred property(i.e., reduction of overall power consumption).

Electron Emission Principle of Electron Emitter

FIG. 8 is a diagram showing the waveform of a drive voltage Va appliedto the electron emitter 120 shown in FIG. 4. FIGS. 9 and 10 each showthe state of operation of the electron emitter 120 of FIG. 4 in the casewhere the drive voltage Va shown in FIG. 8 is applied to the electronemitter 120. Next will be described the principle of electron emissionof the electron emitter 120 with reference to FIGS. 8 to 10.

In the present embodiment, as shown in FIG. 8, the drive voltage Va tobe applied is an alternating voltage of rectangular waveform (period:T1+T2). In the drive voltage Va, the reference voltage (voltagecorresponding to the center of the wave) is 0 V.

As shown in FIGS. 8 to 10, in the drive voltage Va, during time T1corresponding to the first stage, the electric potential of the upperelectrode 124 is V2 (negative voltage), which is lower than the electricpotential of the lower electrode 122; and during time T2 correspondingto the second stage, the electric potential of the upper electrode 124is V1 (positive voltage), which is higher than the electric potential ofthe lower electrode 122.

As shown in FIG. 9A, in the initial state, the emitter section 125 ispolarized unidirectionally, and the negative poles of dipoles facetoward the upper surface 123 a of the emitter layer 123.

Firstly, in the initial state, in which the reference voltage isapplied, as shown in FIG. 9A, the emitter section 125 is polarized suchthat the negative poles of dipoles face toward the upper surface 123 aof the emitter layer 123. In this state, virtually no electrons areaccumulated on the emitter section 125.

Subsequently, as shown in FIG. 9B, when the negative voltage V2 isapplied, polarization is inverted. This inversion of polarization causeselectric field concentration to occur at the aforementioned electricfield concentration points. Through this electric field concentration,electrons are supplied from the electric field concentration points ofthe upper electrode 124 toward the emitter section 125, and then, asshown in FIG. 9C, electrons are accumulated on the emitter section 125.In other words, the emitter section 125 is electrically charged. Thiselectrical charging can be continued until a predetermined saturatedcondition, which depends on the surface resistance of the emitter layer123, is attained. The quantity of the charge can be controlled on thebasis of voltage application time or voltage waveform. Thus, the upperelectrode 124 (in particular, the aforementioned electric fieldconcentration points) functions as an electron supply source for theemitter section 125.

Subsequently, when the drive voltage Va is changed to the referencevoltage as shown in FIG. 10A, and then the positive voltage V1 isapplied as shown in FIG. 10B, polarization is re-inverted. As a result,electrostatic repulsion between the accumulated electrons and thenegative poles of dipoles causes the electrons to be emitted from theemitter section 125 toward the outside of the electron emitter 120through the opening 124 a as shown in FIG. 10C.

In a manner similar to that described above, electrons are emitted fromthe peripheral edge portions 124 b (see FIG. 4) of the upper electrode124.

General Description of Piezoelectric Actuator

FIGS. 11A and 11B are cross-sectional views schematically showing theconfiguration of a piezoelectric actuator 220, which is a dielectricdevice according to the present embodiment. FIGS. 12A and 12B areenlarged cross-sectional views showing essential portions of thepiezoelectric actuator 220 of FIGS. 11A and 11B.

As shown in FIGS. 11A and 11B, a substrate 221, which constitutes thelowermost layer of the piezoelectric actuator 220, is formed of aceramic thin plate. The thickness of the substrate 221 is preferably 3μm to 1 mm, more preferably 5 to 500 μm, particularly preferably 7 to200 μm, from the viewpoints of mechanical strength and the degree ofbending displacement attained by the piezoelectric actuator 220.

As shown in FIGS. 11A and 11B, a cavity 221 a is provided below thesubstrate 221. A small-thickness portion provided above the cavity 221 aforms a vibration plate 221 b which assumes a beam supported at bothends.

A lower electrode 222 is bonded onto the surface of the vibration plate221 b (upper surface in FIGS. 11A and 11B), which is opposite thesurface facing the cavity 221 a. A piezoelectric/electrostrictive layer223, which constitutes the dielectric layer according to the presentinvention, is bonded onto the lower electrode 222. An upper electrode224 is bonded onto the piezoelectric/electrostrictive layer 223.

As shown in FIGS. 11A and 11B, the piezoelectric actuator 220 isconfigured such that when a predetermined drive voltage is appliedbetween the lower electrode 222 and the upper electrode 224 in athickness direction of the piezoelectric/electrostrictive layer 223, thevibration plate 221 b can be bent or deformed through expansion andcontraction of the piezoelectric/electrostrictive layer 223 in adirection perpendicular to the thickness direction, the expansion andcontraction occurring through the transverse piezoelectric effect.

As shown in FIGS. 12A and 12B, in the present embodiment, a portion ofthe piezoelectric/electrostrictive layer 223 on the side of the lowerelectrode 222 (i.e., a lower-electrode-adjacent portion 223 c) and aportion of the piezoelectric/electrostrictive layer 223 on the side ofthe upper electrode 224 (i.e., an upper-surface-adjacent portion 223 d)are formed under the aforementioned predetermined film formationconditions such that each of the portions 223 c and 223 d has apredetermined porosity and contains the aforementioned metal in apredetermined state.

EXAMPLES

Next will be described, with reference to Comparative Examples, Examplesof the aforementioned electron emitter 120 and piezoelectric actuator220, which are dielectric devices according to the present embodiment.

Example 1

The electron emitter 120 of Example 1 (see FIG. 4) employs a zirconiasubstrate serving as a substrate 121. A lower electrode 122 is formed bycoating the substrate 121 with a platinum paste through screen printing,followed by firing of the resultant coating film.

An emitter layer 123 is formed to have a thickness of 10 to 20 μm. Theemitter layer 123 contains PZT (Zr:Ti=52:48) as the aforementioneddielectric material, and silver as the aforementioned metal.

An upper electrode 124 formed of flaky graphite powder is provided onthe emitter layer 123. The upper electrode 124 is formed by coating theemitter layer 123 with a dispersion liquid prepared through dispersionof flaky graphite powder in a solvent containing an organic binder,followed by firing of the resultant coating film.

The aforementioned powdery dielectric material contained in raw materialpowder 81 (see FIG. 3) is PZT (Zr:Ti=52:48) having a number-based meanparticle size of 1.0 μm. The aforementioned fine-particulate metalcontained in the raw material powder 81 is silver having a number-basedmean particle size of 0.35 μm. A film is formed on the substrate 121(accurately, on the lower electrode 122 shown in FIG. 4) from the rawmaterial powder 81 by means of an aerosol deposition apparatus 60, andsubsequently the film is thermally treated for one hour in an electricfurnace, to thereby form the emitter layer 123 shown in FIG. 4.

Table 1 shows the relation between the silver content before thermaltreatment, thermal treatment temperature, silver content and porosityafter thermal treatment, and electron emission efficiency of the emitterlayer 123.

The silver content and porosity after thermal treatment can bedetermined through image analysis of a scanning electron micrograph of across-section of the emitter layer 123.

Electron emission property is obtained as follows.

As shown in FIG. 4, when Va represents drive voltage applied between thelower electrode 122 and the upper electrode 124; Vc represents electronaccelerating voltage (collector voltage) of a bias voltage source 102for generating an external electric field which causes electrons emittedfrom the electron emitter 120 to fly toward a light-emitting panel 101;i_(c) represents current due to the electrons emitted from the electronemitter 120 (i.e., current which flows between the bias voltage source102 and a collector electrode 101 b); and P represents drive power forthe electron emitter 120, electron emission efficiency η is representedby the following formula:η=Vc×i _(c)/(P+Vc×i _(c))(wherein drive power P=[hysteresis loss of electron emitter:P1]+[resistance loss in drive circuit: P2]). P1 is the area enclosed bythe Q-V hysteresis loop shown in FIG. 13 (i.e., the area of the shadedportion shown in FIG. 13). P2, which varies with the method for drivingthe electron emitter, is represented by the following inequality:0≦P2≦(drive voltage Va×electric charge Qe)−(the area enclosed by the Q-Vhysteresis loop)=(the area of a portion outside the shaded portion shownin FIG. 13). In this inequality, 0 on the left side corresponds to thecase where the electron emitter 120 is driven so that the drive powersatisfies the Q-V hysteresis.

TABLE 1 Silver content Thermal Silver content Porosity after beforethermal treatment after thermal thermal Electron treatment temperaturetreatment treatment emission (by volume: %) (° C.) (by volume: %) (%)efficiency 1-1 0 800 0 <0.5 CC 1-2 1 800 0 1 CC 1-3 3 800 0 2 BB 1-4 7700 6 1 CC 1-5 9 750 2 8 BB 1-6 9 800 0 11 AA 1-7 19 700 12 10 DD to CC1-8 19 750 5 17 BB 1-9 19 800 0 20 BB 1-10 32 700 15 18 DD 1-11 32 75010 24 DD to CC 1-12 32 800 0 33 CC

As is clear from Table 1, the electron emitters of Nos. 1-3, 1-5, 1-6,1-8, and 1-9, in which the silver content after thermal treatment is 10%or less and the porosity after thermal treatment falls within a range of2% to 20%, exhibit relatively good electron emission efficiency.

In the electron emitters of Nos. 1-6 and 1-9, the silver content afterthermal treatment is almost 0%. This is because thermal treatment causesmigration of silver in the aforementioned film layer, which constitutesthe emitter layer 123, resulting in localization of silver in thevicinity of the interface between the lowermost part of thelower-electrode-adjacent portion 123 c and the lower electrode 122, orbleeding of silver to a portion above the upper surface 123 a.

In the thus-formed emitter layer 123, lattice defect, lattice strain,etc. are reduced and crystallinity is improved-through thermaltreatment, and the silver content and porosity fall within theabove-described ranges through the aforementioned silver migrationassociated with thermal treatment. Thus, the emitter layer 123 exhibits,for example, dielectric constant and polarization inversion propertysuitable for electron emission.

For example, according to the present Example, there is formed anemitter layer 123 in which the silver content of thelower-electrode-adjacent portion 123 c differs from that of theupper-surface-adjacent portion 123 d, and the porosity is regulated to apredetermined level.

Specifically, for example, there is formed an emitter layer 123 having asilver content gradient in a thickness direction (e.g., the silvercontent decreases from the lower surface 123 b toward the upper surface123 a). Alternatively, there is formed an emitter layer 123 as in thecase of No. 1-6 or 1-9, in which almost the entirety of silver is bledout in the vicinity of the upper surface 123 a. In this case, when theamount of the metal dispersed in the lower-electrode-adjacent portion123 c is regulated to be greater than that of the metal dispersed in theupper-surface-adjacent portion 123 d, the dielectric constant of thelower-electrode-adjacent portion 123 c becomes higher than that of theupper-surface-adjacent portion 123 d. Therefore, electric fieldconcentration occurs in the upper-surface-adjacent portion 123 d. Thiselectric field concentration increases the quantity of electrons to beemitted and improves electron emission efficiency.

In contrast, the electron emitters of Nos. 1-1, 1-2, and 1-4, in whichthe silver content after thermal treatment is 10% or less but theporosity after thermal treatment is less than 2%, are inferior to theaforementioned electron emitters of Nos. 1-3, 1-5, 1-6, 1-8, and 1-9 interms of electron emission quantity and electron emission efficiency(the electron emitter of No. 1-1 corresponds to a conventional electronemitter containing no silver). That is, the electron emitters of Nos.1-1, 1-2, and 1-4 fail to exhibit improvement in electron emissionquantity and electron emission efficiency.

This is considered to be due to insufficient improvement of propertiesof the emitter layer 123 (serving as a dielectric layer) resulting frommigration of silver through thermal treatment and an increase inporosity associated with the silver migration.

Specifically, among the electron emitters of Nos. 1-4, 1-5, and 1-6, thesilver content before thermal treatment (i.e., the amount of silveradded during film formation) is the same, and the thermal treatmenttemperature is different. The higher the thermal treatment temperature,the lower the silver content after thermal treatment, and the higher theporosity. In the electron emitter of No. 1-4, in which the thermaltreatment temperature is relatively low, the silver content afterthermal treatment is not so reduced, and the porosity is not soincreased. In contrast, in the electron emitter of No. 1-5 or 1-6, inwhich the thermal treatment temperature is relatively high, the porosityis increased, and the silver content after thermal treatment is reduced.Thus, in the electron emitter of No. 1-5 or 1-6, conceivably, propertiesof the emitter layer 123 (serving as a dielectric layer) are improved asa result of thermal treatment and silver migration through the thermaltreatment.

The electron emitters of Nos. 1-7 and 1-10, in which the porosity afterthermal treatment falls within a range of 2% to 20% but the silvercontent after thermal treatment exceeds 10%, fail to emit electronsproperly. Specifically, in these electron emitters, unstable electronemission occurs, or short circuit between electrodes occurs frequently.This is considered to be due to impairment of properties of the emitterlayer 123 (serving as a dielectric layer) as a result of a decrease inbreakdown voltage of the entirety of the emitter layer 123 attributed toexcessively high metal content of the emitter layer 123.

Similar to the cases of the electron emitters of Nos. 1-7 and 1-10, theelectron emitters of Nos. 1-11 and 1-12, in which the silver contentafter thermal treatment is 10% or less but the porosity after thermaltreatment exceeds 20%, fail to emit electrons properly. This isconsidered to be due to a decrease in breakdown voltage of the entiretyof the emitter layer 123, and as well impairment of polarizationinversion property.

Example 2

The piezoelectric actuator 220 of Example 2 (see FIG. 11) employs azirconia ceramic substrate serving as a substrate 221. A lower electrode222 is formed by coating the substrate 221 with a platinum paste throughscreen printing, followed by firing of the resultant coating film.

A piezoelectric/electrostrictive layer 223 is formed to have a thicknessof 10 to 20 μm. The piezoelectric/electrostrictive layer 223 containsPZT (Zr:Ti=52:48) as the aforementioned dielectric material, and silveras the aforementioned metal. The piezoelectric/electrostrictive layer223 is formed under conditions similar to those employed in the case ofExample 1.

An upper electrode 224 formed of gold thin film is provided on thepiezoelectric/electrostrictive layer 223. The upper electrode 224 isformed by coating the piezoelectric/electrostrictive layer 223 with agold paste through screen printing, followed by firing of the resultantcoating film.

Table 2 shows the relation between the silver content before thermaltreatment, thermal treatment temperature, and silver content andporosity after thermal treatment of the piezoelectric/electrostrictivelayer 223, and the bending displacement of the piezoelectric actuator220. The bending displacement can be measured by means of a laserDoppler vibrometer.

TABLE 2 Silver content Thermal Silver content Porosity after beforethermal treatment after thermal thermal treatment temperature treatmenttreatment Bending (by volume: %) (° C.) (by volume: %) (%) displacement2-1 0 800 0 <0.5 CC 2-2 1 800 0 1 CC 2-3 3 800 0 2 BB 2-4 7 700 6 1 DDto CC 2-5 10 750 2 9 BB 2-6 10 800 0 13 AA 2-7 21 700 12 12 CC 2-8 21750 5 17 BB 2-9 21 800 0 21 BB 2-10 32 700 13 18 DD 2-11 32 750 10 23 DD2-12 32 800 0 35 DD

As is clear from Table 2, the piezoelectric actuators of Nos. 2-3, 2-5,2-6, and 2-8, in which the silver content after thermal treatment is 10%or less and the porosity after thermal treatment falls within a range of2% to 20%, exhibit relatively good bending displacement.

In contrast, the piezoelectric actuators of Nos. 2-1 (corresponding to aconventional piezoelectric actuator containing no silver), 2-2, and 2-4,in which the silver content after thermal treatment is 10% or less butthe porosity after thermal treatment is less than 2%, are inferior tothe aforementioned piezoelectric actuators of Nos. 2-3, 2-5, 2-6, and2-8 in terms of bending displacement. That is, the piezoelectricactuators of Nos. 2-1, 2-2, and 2-4 fail to exhibit improvement inbending displacement.

In the piezoelectric actuators of Nos. 2-7 and 2-10, in which theporosity after thermal treatment falls within a range of 2% to 20% butthe silver content after thermal treatment exceeds 10%, short circuitbetween electrodes occurs frequently, and proper bending displacementfails to be attained. This is considered to be due to impairment ofproperties of the piezoelectric/electrostrictive layer 223 (serving as adielectric layer) as a result of a decrease in breakdown voltage of theentirety of the piezoelectric/electrostrictive layer 223 attributed toexcessively high metal content of the piezoelectric/electrostrictivelayer 223.

Similar to the cases of the piezoelectric actuators of Nos. 2-7 and2-10, in the piezoelectric actuators of Nos. 2-11 and 2-12, in which thesilver content after thermal treatment is 10% or less but the porosityafter thermal treatment exceeds 20%, proper bending displacement failsto be attained. This is considered to be due to a decrease in withstandvoltage of the entirety of the piezoelectric/electrostrictive layer 223,and as well impairment of piezoelectric properties.

The piezoelectric actuator of No. 2-9, in which the porosity afterthermal treatment slightly exceeds 20% but the silver content afterthermal treatment is almost 0%, exhibits relatively good bendingdisplacement. The results imply that piezoelectric properties of thepiezoelectric/electrostrictive layer 223 are affected not by theporosity of the layer, but mainly by migration of silver (theaforementioned metal) in the layer and the silver content of the layer.

Example 3

The electron emitter 120 of Example 3 (see FIG. 4) employs a soda glasssubstrate serving as a substrate 121. A lower electrode 122 and an upperelectrode 124 are formed in a manner similar to that of Example 1.

An emitter layer 123 is formed to have a thickness of 10 to 20 μm. Theemitter layer 123 contains PZT (Zr:Ti=52:48) as the aforementioneddielectric material, and silver as the aforementioned metal.

In the present Example, the emitter layer 123 is formed by means of anaerosol deposition apparatus 60 as shown in FIG. 14. The aerosoldeposition apparatus 60 includes a deposition chamber 70. The depositionchamber 70 includes a nozzle 73 and an additional nozzle 73′ therein.

The aerosol deposition apparatus 60 includes an aerosol supply unit 80and an additional aerosol supply unit 90. The aerosol supply unit 90 hasthe same configuration as the aerosol supply unit 80, and is configuredsuch that an aerosol containing raw material powder 91 can be suppliedto the nozzle 73′. The aerosol supply unit 90 includes a second aerosolchamber 92, a compressed gas supply source 93, a compressed gas supplytube 94, a vibration stirring section 95, an aerosol supply tube 96, anda control valve 97.

In the aerosol deposition apparatus 60 shown in FIG. 14, the metalcontent of the raw material powder 81 differs from that of the rawmaterial powder 91. Specifically, the aerosol deposition apparatus 60 ofFIG. 14 is configured such that the raw material powder 81 and the rawmaterial powder 91, which are different in the ratio of incorporateddielectric material powder to incorporated fine metal particles, can beseparately sprayed through the nozzle 73 and the nozzle 73′,respectively.

When, in the aerosol deposition apparatus 60, spraying through thenozzle 73 and spraying through the nozzle 73′ are appropriately switchedwith each other, while the raw material powder 81 contained in theaerosol chamber 82 or the raw material powder 91 contained in theaerosol chamber 92 is appropriately replaced, a film is formed so thatthe ratio of dielectric material powder to fine metal particles isgradually varied in a thickness direction of the film. The thus-formedfilm layer is thermally treated in a manner similar to that describedabove. This procedure yields an emitter layer 123 in which the silvercontent of a lower-electrode-adjacent portion 123 c differs from that ofan upper-surface-adjacent portion 123 d. Specifically, in the presentExample, the emitter layer 123 is formed so that the metal content ofthe upper-surface-adjacent portion 123 d is lower than that of thelower-electrode-adjacent portion 123 c.

Table 3 shows the relation between the silver content of thelower-electrode-adjacent portion 123 c after thermal treatment, thesilver content of the upper-surface-adjacent portion 123 d after thermaltreatment, and the electron emission efficiency of the emitter layer123.

The silver content of the lower-electrode-adjacent portion 123 c(corresponding to “lower layer” described in Table 3) can be determinedthrough image analysis of a scanning electron micrograph of across-section of a portion of the emitter layer 123, the portion beinglocated in the vicinity of the interface between the lower electrode 122and the emitter layer 123. Similarly, the silver content of theupper-surface-adjacent portion 123 d (corresponding to “upper layer”described in Table 3) can be determined through image analysis of ascanning electron micrograph of a cross-section of a portion of theemitter layer 123, the portion being located below the lowermost end ofconcavities 123 a 1 formed on the upper surface 123 a of the emitterlayer 123.

TABLE 3 Silver content after Silver content after Electron thermaltreatment thermal treatment emission (lower layer: vol %) (upper layer:vol %) efficiency 3-1 0 0 CC 3-2 2 0 BB 3-3 11 0 AA 3-4 18 0 BB 3-5 22 0CC 3-6 8 2 BB 3-7 19 4 BB 3-8 20 7 DD

As is clear from Table 3, as compared with the electron emitter of No.3-1 (corresponding to a conventional electron emitter containing nosilver), the electron emitters of Nos. 3-2, 3-3, 3-4, 3-6, and 3-7, inwhich the silver content of the lower-electrode-adjacent portion 123 c(corresponding to “lower layer” described in Table 3) falls within arange of 2% to 20%, and the silver content of the upper-surface-adjacentportion 123 d (corresponding to “upper layer” described in Table 3) is5% or less, exhibit relatively good electron emission efficiency.

In the electron emitters of Nos. 3-2, 3-3, 3-4, 3-6, and 3-7, whichexhibit good properties, the metal content of the upper-surface-adjacentportion 123 d is ¼ or less that of the lower-electrode-adjacent portion123 c. Particularly, in the electron emitters of Nos. 3-2, 3-3, and 3-4,the metal content of the upper-surface-adjacent portion 123 d is almostzero.

Thus, in the present Example, when the amount of the metal dispersed inthe lower-electrode-adjacent portion 123 c is regulated to be greaterthan that of the metal dispersed in the upper-surface-adjacent portion123 d, the dielectric constant of the lower-electrode-adjacent portion123 c becomes higher than that of the upper-surface-adjacent portion 123d. Therefore, electric field concentration occurs in theupper-surface-adjacent portion 123 d. This electric field concentrationincreases the quantity of electrons to be emitted and improves electronemission efficiency.

In contrast to these electron emitters, in the electron emitter of No.3-5, in which the silver content of the lower-electrode-adjacent portion123 c exceeds 20%, dielectric breakdown occurs, and proper electronemission fails to be attained. Similar to the case of the electronemitter of No. 3-5, in the electron emitter of No. 3-8, in which thesilver content of the upper-surface-adjacent portion 123 d exceeds 5%,proper electron emission fails to be attained.

Example 4

The piezoelectric actuator 220 of Example 4 (see FIGS. 11 and 12) isformed in the same manner as that of Example 2, except that apiezoelectric/electrostrictive layer 223 is formed under conditionssimilar to the film formation conditions employed in Example 3.

Table 4 shows the relation between the silver content of alower-electrode-adjacent portion 223 c of thepiezoelectric/electrostrictive layer 223 after thermal treatment, thesilver content of an upper-surface-adjacent portion 223 d of the layer223, and the bending displacement of the piezoelectric actuator 220.

TABLE 4 Silver content after Silver content after thermal treatmentthermal treatment Bending (lower layer: vol %) (upper layer: vol %)displacement 4-1 0 0 CC 4-2 2 0 BB 4-3 11 0 BB 4-4 18 0 BB 4-5 21 0 CC4-6 28 0 DD 4-7 18 3 BB 4-8 20 7 DD

As is clear from Table 4, as compared with the piezoelectric actuator ofNo. 4-1 (corresponding to a conventional piezoelectric actuatorcontaining no silver), the piezoelectric actuators of Nos. 4-2, 4-3,4-4, and 4-7, in which the silver content of thelower-electrode-adjacent portion 223 c (corresponding to “lower layer”described in Table 4) falls within a range of 2% to 20%, and the silvercontent of the upper-surface-adjacent portion 223 d (corresponding to“upper layer” described in Table 4) is 5% or less, exhibit relativelygood bending displacement.

In the piezoelectric actuators of Nos. 4-2, 4-3, 4-4, and 4-7, whichexhibit good properties, the metal content of the upper-surface-adjacentportion 223 d is ¼ or less that of the lower-electrode-adjacent portion223 c. Particularly, in the piezoelectric actuators of Nos. 4-2, 4-3,and 4-4, the metal content of the upper-surface-adjacent portion 223 dis almost zero.

Thus, in the present Example, when the amount of the metal dispersed inthe lower-electrode-adjacent portion 223 c is regulated to be greaterthan that of the metal dispersed in the upper-surface-adjacent portion223 d, the dielectric constant of the lower-electrode-adjacent portion223 c becomes higher than that of the upper-surface-adjacent portion 223d. Therefore, electric field concentration occurs in theupper-surface-adjacent portion 223 d. This electric field concentrationincreases the amount of deformation of the upper-surface-adjacentportion 223 d through the transverse piezoelectric effect, and attainslarger bending displacement.

In contrast to these piezoelectric actuators, in the piezoelectricactuators of Nos. 4-5 and 4-6, in which the silver content of thelower-electrode-adjacent portion 223 c exceeds 20%, good bendingdisplacement fails to be attained. Meanwhile, in the piezoelectricactuator of No. 4-8, in which the silver content of theupper-surface-adjacent portion 223 d exceeds 5%, proper electronemission fails to be attained.

Modifications

The aforementioned embodiment and Examples are merely typical embodimentand Examples of the present invention which have been considered best bythe present applicant at the time when the present application has beenfiled. Thus, the present invention is not limited to the aforementionedembodiment and Examples. Therefore, it should be understood that variousmodifications of the aforementioned embodiment and Examples may be madeso long as the essentials of the present invention are not changed.

Several typical modifications will next be described. In thebelow-described modifications, members having configuration and functionsimilar to those described in the aforementioned embodiment and Examplesare denoted by the same reference numerals as those employed in theembodiment and Examples. Description of the embodiment and Examples canbe applied to description of such members, so long as these descriptionsdo not technically contradict each other.

Needless to say, modifications of the aforementioned embodiment andExamples are not limited to the below-described ones. Meanwhile, aplurality of modifications may be appropriately employed in combination,so long as these modifications do not technically contradict oneanother.

(i) Application of the present invention is not limited to electronemitters and piezoelectric actuators as described above in theembodiment and Examples. The present invention can be suitably appliedto, for example, ceramic filters (e.g., an SAW filter), piezoelectricbuzzers, piezoelectric vibratory gyroscopes, piezoelectric microphones,various piezoelectric sensors, and Rosen-type piezoelectrictransformers.

(ii) Application of the electron emitter 120 according to theaforementioned embodiment and Examples is not limited to FEDs. Theconfiguration of the electron emitter 120 is not limited to thatdescribed in the aforementioned embodiment and Examples.

For example, in the electron emitter 120 according to the aforementionedembodiment, the lower electrode 122 is formed on the lower surface 123 bof the emitter layer 123, and the upper electrode 124 is formed on theupper surface 123 a of the emitter layer 123. However, thisconfiguration may be modified such that an electrode other than theupper electrode 124 is formed on the upper surface 123 a of the emitterlayer 123. In this case, the aforementioned drive voltage is appliedbetween the upper electrode 124 and the electrode other than the upperelectrode 124.

The substrate 121 may be formed of a metal in place of a glass orceramic material.

(iii) The material of the dielectric layer 13 (the emitter layer 123 orpiezoelectric/electrostrictive layer 223) or the form of theaforementioned metal contained in the layer may be appropriatelymodified. For example, the metal may be contained in the form of oxide.

The state of distribution of the aforementioned metal may beappropriately modified within the scope of the present invention. Forexample, each of the lower-electrode-adjacent portion 13 c (thelower-electrode-adjacent portion 123 c or 223 c) and theupper-surface-adjacent portion 13 d (the upper-surface-adjacent portion123 d or 223 d) may be formed so as to have a thickness about ½ that ofthe dielectric layer 13 (the emitter layer 123 orpiezoelectric/electrostrictive layer 223).

Alternatively, the lower-electrode-adjacent portion 13 c (thelower-electrode-adjacent portion 123 c or 223 c) and/or theupper-surface-adjacent portion 13 d (the upper-surface-adjacent portion123 d or 223 d) may be formed so as to have a very small thickness;i.e., a thickness about 1/10 that of the dielectric layer 13 (theemitter layer 123 or piezoelectric/electrostrictive layer 223).

(iv) Operational and functional elements constituting means forachieving the objects of the present invention encompass, in addition tospecific structures disclosed in the aforementioned embodiment,Examples, and modifications, any structure capable of attaining theoperation and function of the present invention.

1. A dielectric device comprising: a substrate; a base electrode whichis formed of a conductor film and is provided on the substrate; and adielectric layer formed on the base electrode, said dielectric layercomprising free metal, wherein the dielectric layer is formed so thatthe free metal concentration of a first portion of the dielectric layerlocated between the center of the dielectric layer in a thicknessdirection thereof and the base electrode is higher than that of a secondportion of the dielectric layer located between the center of thedielectric layer and an upper surface thereof, and wherein the metalconcentration by volume of the first portion is 2% to 20%, and the metalconcentration by volume of the second portion is 5% or less.
 2. Adielectric device according to claim 1, wherein the dielectric layer isformed so that the porosity of the dielectric layer falls within a rangeof 2% to 20%.